That's interesting to know.
It's also worth noting that technically Pitt isn't a public school. It's actually a private school "with state support." Which means that it's a private school that, a long time ago, made agreements with the commonwealth of PA b/c Pittsburgh was one of the biggest cities in the country (at the time) and didn't have a good public university. This is why, BTW, Temple is cheaper than Pitt and Pitt's undergraduate is a bit more expensive (around 10k) than most public colleges.
But it's relevant b/c Pitt isn't required to follow any requirement or guideline for how many state residents they must admit -- as many other public schools are (wisconsin, iowa, texas, virginia, tennessee -- the list goes on).
You're kinda right. Pitt is a public university. It just isn't a state school. Neither is Penn State, actually, though the name would deceive one into thinking that. The standard way of talking about the schools is that they are "state-funded national universities." So Pitt's non-requiurement to accept in-staters isn't because they are private, but because they are not a state school (state schools in PA would Edinboro, Slippery Rock, Clarion, Kutztown, etc).